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‘Molecular computers’ act as tiny ID tags

By Kurt Kleiner

Molecules capable of basic logic operations have been developed that could serve as tiny ID tags for identifying individual cells or nano-devices. The technique, called molecular computational identification (MCID), could produce tens of millions of unique tags.

The idea comes from research on molecules that work like silicon logic gates. Created by Prasanna de Silva and colleagues at Queen’s University in Belfast, UK, the molecules use the presence of a chemical, or a mix of chemicals, as inputs, and give off light as output.

Simple “YES” and “NOT” gates either light up, or not, depending on the presence of a single chemical. A “NOR” gate lights up if neither of two chemicals are present, while an “AND” gate lights up only if two chemicals are present.

The promise of molecular computation is that it would allow billions of calculations to occur simultaneously in a test tube. But stringing the simple operations together to allow complex operations has been difficult, de Silva admits, and he wanted to find an immediate use for the computational molecules. So he came up with a method to use computational molecules as molecular tags.

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Molecular ID tags are analogous to “radio frequency identification” (RFID) tags based on silicon chips. RFID tags are tiny chips that broadcast a unique ID when queried. The smallest are just 0.3 millimetres to a side.

However, that is still too big for some applications, such as trying to attach a tag to an individual cell, which could be useful in medical research. This gave de Silva hope that the molecular approach had promise.

To test the theory, de Silva and colleagues attached five different “logic molecules” to polystyrene beads, treated them with chemicals, and showed they could be identified under a microscope.

“Through the roof”

Eventually the process could be automated, and the combinations of molecules read off like numbers from a license plate, de Silva says. Tagging individual cells would allow each to be tracked as it passed through a “lab-on-a-chip” for example.

“What really makes the numbers go through the roof is combining operations,” de Silva says. By combining the logical functions, large numbers of individual tags can be created that each give a different output. The beads can then be “washed” in the various chemicals that serve as inputs, and the answer can be read out in the form of fluorescence.

The tags might be used in medical research, allowing researchers to tag and identify individual cells, for instance, which would fluoresce in the presence of a specific chemical. It might also be useful for nanotechnologists who need to keep track of thousands or millions of tiny nanostructures.

“The study shows that molecular computation can indeed find real applications today,” says Vincenzo Balzani, a chemist at the University of Bologna in Italy.